To function properly, many high temperature electrochemical devices, such as ceramic-based fuel cells, oxygen generators, and chemical sensors, often require metal and ceramic components to be hermetically sealed each other. Unfortunately, the chemical and physical characteristics of many of the ceramic and metal components used in these devices have presented a variety of challenges for the development of effective seals. For example, one standard electrolyte material currently employed in nearly all of these devices is yttria stabilized zirconia (YSZ) because of its excellent oxygen ion transport properties, insulating electronic nature, and exceptional chemical stability under a wide variety of operating conditions and environments. However, to generate a sufficiently high rate of ionic transport, the device must be operated at high temperature, typically on the order of 650–900° C., and the thickness of the electrolyte membrane must be minimized; though generally no thinner than 5–10 μm, to mitigate the formation of through-thickness pinhole defects during manufacture. Since a solid state electrochemical device such as a fuel cell functions due to the oxygen ion gradient that develops across the electrolyte membrane, not only is hermiticity across the membrane important, but also that across the seal which joins the electrolyte to the body of the device. That is, the YSZ layer must be dense, must not contain interconnected porosity, and must be connected to the rest of the device with a high temperature, gas-tight seal. Typical conditions under which these devices are expected to operate and to which the accompanying YSZ-to-metal joints will be exposed include: 1) an average operating temperature of 750° C.; 2) continuous exposure to an oxidizing atmosphere on the cathode side; and 3) an anticipated device lifetime of 3000–30,000+ hours, as defined by the specific application. Depending on the function of the device, e.g. energy generation, the seal may also be exposed to a reducing environment on the anode side.
One approach to bonding metal with YSZ for operation in such environments, active metal brazing, utilizes a braze alloy that contains one or more reactive elements, often titanium, which will chemically reduce the ceramic faying surface and greatly improve its wetting behavior and adherence with the braze. However, there are at least two problems with using this type of joining material in fabricating solid-state electrochemical devices: 1) the complete oxidation of the active species in the braze during high temperature operation of the device will often lead to rapid deterioration of the joint at the ceramic/braze metal interface and an eventual loss in hermeticity and 2) exposure of the entire device to a reducing atmosphere at a temperature greater than ˜800° C., typical processing conditions for active metal brazing, is often too demanding for many of the complex oxide materials used in these devices. When employed as electrochemically active electrodes, these mixed ionic/electronic conducting oxides tend to reduce during the joining operation and may irreversibly deteriorate via phase separation, which ultimately causes severe degradation in device performance. Thus, there exists a need for new methods of forming seals that overcome these difficulties and produce metal to ceramic seals which function satisfactorily in these demanding environments.